Viral transcriptional activators have been important tools to elucidate the mechanisms of transcriptional regulation in animal cells. We have been studying two viral activators adenovirus (Ad) E1a and Herpes Simplex Virus (HSV) VP16. These two proteins are essential for viral replication and function by stimulating transcription of specific viral genes. Over the past period of funding we have made significant progress in understanding the mechanisms by which these two viral proteins function through cellular factors to activate transcription. The HSV VP16 protein contains the prototype "acidic" activation domain. Over the past period of funding we have found that an important aspect of VP16 function is a direct interaction with the general transcription factor (GTF) TFIIB. The VP16-TFIIB interaction facilitates the recruitment of TF11B into the preinitiation complex. Experiments are proposed to understand the VP16-TFIIB interaction, and its role in transcription activation, in further detail. Like VP16, the 289aa Ad E1a protein contains an activation region. Several observations suggest that E1a function is mediated, at least in part, through interaction with an unidentified cellular co-activator. Experiments are proposed to identify, isolate and clone this cellular component. E1a is not a sequence-specific DNA binding protein, and over the past period of funding we have found that E1a is targeted to promoters through interaction with several types of cellular DNA binding domains. We will perform experiments to clarify how interaction with apparently diverse protein motifs is achieved. Through studies of E1a activation we discovered the ATF family of cellular transcription factors, which are involved in the expression of a variety of viral and cellular activation by several other, diverse agents. Experiments are proposed to understand in greater detail regulation of ATF-2 transcriptional activity.